1. Field of the Invention
The present invention relates to selective reduction of NOx, typically contained in exhaust gas from diesel engines, by NH.sub.3 that is generated by decomposition of chemicals such as urea, solutions of urea in water or solutions of NH.sub.3 in water. The NH.sub.3 is generated in the gas phase upstream of the so-called SCR catalyst for the selective reduction of NOx.
2. Description of the Related Art
The chemical used as reductant is injected in liquid form as a spray, generated in a spray nozzle, into the exhaust gas stream having a temperature of typically 300-400.degree. C. as required for the catalytic NOx reduction process. The droplets must, however, be evaporated and urea decomposed into NH.sub.3, CO.sub.2 and H.sub.2 O before the NH.sub.3 can be utilized in the SCR catalyst for the NOx-reduction process. Evaporation and decomposition of the droplets is strongly endothermal.
The rate of evaporation and decomposition (gasification) of the droplets is limited by the rate at which heat is transferred from the hot gas to the surface of the droplets through the gas film surrounding the droplets. This means that the time for gasifying a droplet (the residence time) can be estimated by the equation: EQU t=K.times.d.sup.-2 .times.(T-T.sub.min) seconds
where d is the (initial) diameter of the droplet, T is the temperature of the gas, and T.sub.min is the minimum temperature required for decomposition of the urea at an appropriate rate (i.e., about 180.degree. C.), while K is a constant that is nearly independent of the gas velocity.
Thus, the residence time of the droplets increases strongly with the size of the droplets.
The distance between the injection nozzle and the SCR catalyst required for achieving complete gasification of the droplets in the gas stream, furthermore, increases proportionally to the gas velocity in the duct.
It is estimated from experiments and calculations that a residence time of 100 .mu.m droplets of a 35% solution of urea in exhaust gas at 300.degree. C. is on the order of 1 second, which means that a distance of 20 m from the nozzle to the SCR catalyst is required for complete gasification of the droplets at 20 m/s gas velocity in the duct.
On the other hand, if only 0.5 m distance from injection nozzle to the SCR catalyst is available (as in the case of diesel vehicles), it is necessary to reduce the size of the droplets to less than 8 .mu.m in order to achieve complete gasification of the droplets in the gas phase upstream of the SCR catalyst. Such a size reduction is very difficult to achieve. Moreover, without such a reduction, most of the particles or droplets in the gas will pass unreacted through the SCR catalyst when the catalyst is monolithic with straight channels throughout the catalytic block.
It has surprisingly been found that relatively large droplets of reductant generated by a simple spray nozzle can be quickly and completely gasified if the droplets are caused to impinge on surfaces in a device that can easily be installed in the limited distance available between the nozzle and the SCR catalyst in, e.g., a diesel vehicle.